Mutation rates among RNA viruses

Abstract

The rate of spontaneous mutation is a key parameter in modeling the genetic structure and evolution of populations. The impact of the accumulated load of mutations and the consequences of increasing the mutation rate are important in assessing the genetic health of populations. Mutation frequencies are among the more directly measurable population parameters, although the information needed to convert them into mutation rates is often lacking. A previous analysis of mutation rates in RNA viruses (specifically in riboviruses rather than retroviruses) was constrained by the quality and quantity of available measurements and by the lack of a specific theoretical framework for converting mutation frequencies into mutation rates in this group of organisms. Here, we describe a simple relation between ribovirus mutation frequencies and mutation rates, apply it to the best (albeit far from satisfactory) available data, and observe a central value for the mutation rate per genome per replication of micro(g) approximately 0.76. (The rate per round of cell infection is twice this value or about 1.5.) This value is so large, and ribovirus genomes are so informationally dense, that even a modest increase extinguishes the population.

Figure 1

The accumulation of mutations during a single round of infection. The diagram shows the consequences of an arbitrary μ_g = 0.2 at both the first and second rounds, and the scheme is simplified by setting n₂ = 20 for all second rounds of copying. The burst size is n₁n₂ = 100. ○, unmutated genomes; ➊, genomes with a single mutation; ➋, genomes with two mutations, which may arise in sequential replications (as in the fourth line) or during a single replication (as in the bottom line).